The present invention relates to radio frequency (RF) systems used for navigational purposes, and, more particularly, to RF receivers which operate in satellite navigation systems and are structured with diverse anti-jam capabilities to enable receiver and navigation operations in a hostile jammer environment.
Satellite navigation systems employ a plurality of satellites which generate respective signals for reception by an RF receiver on an airborne or surface vehicle or other platform for computation of the platform position. A navigation control can thus steer the platform in its motion on the basis of a stream of position calculations. The United States and Russia recently deployed satellite RF navigation systems respectively called the Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS). For a detailed description of these navigation systems, reference is made to a book entitled "Understanding GPS: Principles and Application", edited by E. D. Kaplan, and published in 1996 by Artech House Publishers of Boston, Mass.
In this disclosure, the term GPS refers generically to satellite navigation systems including the Global Position System, GLONASS, and any additional satellite navigation system which may be created in the future such as the system planned by the European Community.
The satellite navigation systems, now deployed, were developed with civilian access built into the signal structure, and these systems are thus highly vulnerable to electronic attack measures. The same is likely to be true for any future navigation system.
To date, the United States government has examined a number of techniques for ensuring RF navigation in a jamming environment. These anti-jam techniques have been independently developed with little or no strategic plan as to how, when, or where these various techniques may be used. Table 1 provides an overview of capabilities and usage of existing RF navigation related anti-jam technologies.
TABLE 1 ______________________________________ OFF-THE-SHELF ANTI-JAM CAPABILITIES Anti-Jam Typical Anti- Technology Jam Capability Use Where ______________________________________ Antennas with -- Multipath Antenna Choke Rings Mitigation Upwards Looking 30 to 60 dB Eliminate Ground Antenna Antennas Level Jammers Fixed Reception 25 to 35 dB Fixed Jammers Antenna Pattern Antenna Fixed Nulls (FRPA) Controlled 25 to 35 dB Mobile Jammers Antenna Reception Steerable Pattern Antenna Nulls (CRPA) Cavity Filters -- Eliminate Out-of- Pre- or Bandpass band Interference Filtering Pre-filters Notch Filters 20 to 30 dB CW Jamming RF Filtering Transversal 20 dB (PSK) to CW & PSK Jamming IF Filters 30 dB (CW) Filtering Jammer-to-Noise -- Detect Jammers Digital Power Ratio Receiver Meters Non-linear A/D -- Limit Interference Digital Converters Receiver Wider Dynamic .about.6 dB per Bit Any Jammers Digital Range A/D Receiver Converters Anti-Spoofing -- Limit Access to Digital (A-S) encrypted P-code Receiver Y-code INS Aiding of .about.20 dB Coasting through Digital Code/Carrier Jamming Signals Receiver Loops Parallel -- Direct Y-code Digital Correlators Acquisition Receiver ______________________________________
Even in a jammer-free and interference-free environment, the navigational accuracy of existing military RF receivers is inadequate for certain weapon-platforms. For example, such RF receivers may produce unacceptable vertical position errors in weapon delivery. A need thus exists for a GPS receiver which provides greater navigational accuracy.
In an RF environment, interference signals may have jamming effects on an RF receiver even though they are not intentionally generated as jamming signals by combatants in a military environment or by terrorists in a civil or commercial environment. Jamming interference signals may originate, for example, in a laptop computer on an airplane, a radio or television tower, or any high frequency device such as a radar, a radio, or a cellular phone.
Accordingly, the term "jammer signal" herein refers to both jammer and interference signals. In illustration, the term "jammer free" includes "interference-free", and the term "anti-jam capability" includes "anti-interference" capability.
With the removal of selective availability, the use of various forms of jamming can provide a means for maintaining an edge in RF navigation in theaters of war. In turn, the use of electronic attack (jamming) against the use of GPS navigation technology leads to a need for electronic protection (anti-jam technology) which enables use of the RF navigation technology in a jamming environment.
In state-of-the-art GPS receivers having anti-jam capability, the magnitude of jammer suppression has been limited by limited anti-jam capabilities. Moreover, existing design concepts have typically limited the kinds of jammers which can be suppressed in such GPS receivers. It is desirable that a basic GPS receiver structure be developed to provide better jammer suppression and to provide suppression of a wide range of different kinds of jammers. Further, a need exists for GPS receiver structures used in a variety of platforms, such as airplanes, land vehicles, cruise missiles or smart munitions, manportable units, ordinary munitions, etc. Accordingly, a wide variety of anti-jam requirements exists for the wide range of possible applications.
With previously known GPS anti-jam receivers, this variety of requirements can be met only by a costly proliferation of GPS designs for the different platform applications. It is thus desirable that a basic GPS receiver structure be developed to enable anti-jam capabilities to be provided cost effectively over a wide range of platform applications.
In one particular aspect of state-of-the-art GPS receivers, an analog/digital interface is placed as close as possible to the GPS receiver antenna to obtain an anti-jam capability principally through digital circuitry functioning with or without antenna nulling. This prior art analog/digital design concept has actually limited anti-jam capabilities in GPS receivers, as more fully explained subsequently hereinafter, and typically has employed a multitude of deep antenna nulls which can be substantially reduced in effectiveness by "toggling" jammers operating to confuse the anti-jamming operation of the deep nulls.
Other prior art RF systems, such as Electronic Support Measures (ESM) and anti-radiation missile seekers (ARM) used for navigation and targeting purposes have also been limited in performance similar to the described limitations of prior art GPS RF receivers.
Accordingly, the present invention is directed to meeting existing needs in the pertaining art and to overcoming the described prior art limitations.